1. Field of the Invention
The present invention generally relates to extrudable adhesive blends and to composite structures incorporating these adhesive blends. Useful multi-layer structures of the invention comprise at least one layer of 4-methylpentene-1 polymer having a barrier layer of vinyl alcohol polymer on at least one side and an adhesive interlayer sandwiched therebetween.
2. Description of the Prior Art
Crystalline homopolymers and copolymers of 4-methylpentene-l(4MP1), generally referred to herein as polymethylpentenes (PMPs), are known and are commercially available. Whereas PMPs have mechanical properties comparable to other olefin polymers, such as polypropylene, their thermal properties are similar to engineering plastics. This combination of mechanical and thermal properties coupled with superior chemical resistance and high optical transparency makes PMPs a highly useful class of general purpose plastics. PMPs can be extruded to produce films, sheets and laminated structures--generally using the same processing equipment as employed for polyolefins but varying the operating conditions.
Owing to their superior properties and high melting and softening points, homopolymers and copolymers of 4MP1 are particularly well suited for the construction of laminated containers for use with heated food products. For example, coextruded laminates used to produce grease resistant dual ovenable paperboard based structures having a PMP food contact resin layer are disclosed in U.S. Pat. No. 5,002,833. The disclosed structures are suitable for use in both conventional and microwave ovens and two specific laminate structures are disclosed--the first comprising a paperboard substrate having a five-layer coating coextruded thereon (paperboard/PMP/tie/barrier/tie/PMP) and the second having a three-layer coating coextruded onto paperboard (paperboard/barrier/tie/PMP). The PMP resin layer imparts good grease and oil resistance, superior food release properties and resistance to warpage. Polyamides, copolyamides, polyesters and copolyesters are indicated to be useful barrier layers and the tie layer is a chemically modified graft copolymer of methylpentene.
Since ethylene-vinyl alcohol (EVOH) copolymers are highly effective barrier resins, it would be useful if composite structures comprised of one or more PMP layers with an EVOH barrier layer were available. The ability to produce such structures has, however, been restricted due to the limited availability of extrudable adhesives that can be interlayered between the PMP and EVOH and which have strong interfacial adhesion to both the PMP and the EVOH.
An adhesive blend suitable for laminating PMPs and thermoplastic resins, including EVOH, has been disclosed in U.S. Pat. No. 5,106,692. The blends are comprised of 40 to 98% by weight ethylene/.alpha.-olefin random copolymer containing 30 to 95 mole % ethylene units, 0.1 to 20% by weight polyolefin modified with an unsaturated carboxylic acid or its acid halide, amide, imide or anhydride, and 1 to 50% by weight tackifier. Tackifiers used for the blends are typically hydrocarbon resins produced from various fractions obtained from petroleum and naphtha cracking operations.
While the three-component adhesive blends of U.S. Pat. No. 5,106,692 can be used to effectively adhere PMP to EVOH, it would be highly desirable if other adhesives were available for this purpose. It would be advantageous if the adhesive blends utilized fewer than three components since this would simplify the blending operation and minimize the opportunity for error. Also, processors who formulate their own adhesive blends could inventory fewer materials. It would be even more advantageous if the adhesive blends did not require use of a hydrocarbon resin tackifier since these products can impart undesirable color and can adversely affect theological properties. Furthermore, many hydrocarbon resin tackifiers do not have acceptable FDA clearance for the desired applications. These and other advantages are realized with the two-component adhesive blends of the present invention which are described in more detail to follow.